1. Field of the Invention
While the invention is of general utility, it is particularly suitable for use with, and will be described in connection with, the apparatus of the type shown and described in copending patent application U.S. Ser. No. 240,700 for Two-Lead Electrical Control Apparatus, assigned to the same assignee, and now allowed.
The present invention relates to thermostat controlled heating systems, and more particularly to electrical circuits which allow for a larger fuel flow rate during ignition in comparison with the rate necessary to maintain the burn.
2. Description of the Prior Art
In gas fueled home heating installations and similar facilities a typical problem is that the gas-air mixture necessary to achieve ignition of the burner chamber or cavity is well defined and steps must be taken to assure repeatability in attaining a mixture near the ignitor which will ignite. In view of the transient fuel flow nature, local variations occur which at low velocities of fuel flow in the burner cavity may be locally insufficient for ignition. Because of the small size of a typical ignitor it is desirable to have maximum ignitable mix in the chamber to insure contact between the mix and the ignitor. Most prior art systems did not discriminate between the ignition or start up state and the burn state, particularly since the burn state typically also calls for some fairly high fixed level of fuel supply to provide enough heating. Both the ignition and burn requirement has all of the characteristics of a contactor system, i.e., an on-off system which is very often incompatible with precise control over temperature.
In phase commutated prior art systems, such as the one described in U.S. Pat. No. 3,818,247, gain linearity is achieved by truncating the height of the commutated signal to a fuel control valve. Such has been done in the above-referenced patent by a Zener diode connected across the control valve, such Zener diode effectively converting phase commutation to pulse width modulated type of control. In this manner, signal levels above Zener breakdown were left unutilized.
Once the system is ignited, however, the criteria for maintaining combustion are much less stringent and such lower signal levels and consequently lower fuel flow rates can be applied in order to sustain combustion.